In a nuclear power plant, in general, after a reactor is operated for a fixed period, a spent fuel removed out of the reactor core is temporarily stored in a spent fuel storage pool before the spent fuel is reprocessed. In the spent fuel storage pool, the spent fuel is subjected to cooling and decay heat removal process.
The spent fuel is stored in a spent fuel storage rack having a grid structure, and such grid structure and layout thereof are determined from the viewpoint of ensuring a subcritical state. Specifically, cells made of boron stainless steel containing boron, which excels in absorbing neutrons, are disposed in a grid pattern, and upper and lower portions of the cells are positioned by using grid-shaped frame plates so as to keep constant the interval between the cells.
In the thus configured spent fuel storage rack of prior art, since the upper and lower grid-shaped frame plates need to withstand any load at the time of earthquake, the spent fuel storage rack has, for example, a structure in which wall plates are provided on the four outer surfaces of the rack to support the upper and lower grid-shaped frame plates, or a structure in which the sides of the upper and lower grid-shaped frame plates that face pool walls are connected to the pool walls and the sides of the upper and lower grid-shaped frame plates that face an adjacent spent fuel storage rack are connected to the corresponding sides of the adjacent rack to support the upper and lower grid-shaped frame plates.
Furthermore, in a conventional technology, as a neutron absorber, boron stainless steel having a boron density of about 1% or lower is utilized, and each cell made of the boron stainless steel has been a pipe formed in a drawing or welding process.